ci: Add a sleep test

I think this catches errors in notification for the Linux backend.

Signed-off-by: John Nunley <dev@notgull.net>

tests/sleep.rs

@@ -0,0 +1,13 @@
+//! Sleep test.
+
+use async_process::Command;
+use futures_lite::future::block_on;
+
+#[cfg(unix)]
+#[test]
+fn unix_sleep() {
+    block_on(async {
+        let status = Command::new("sleep").arg("1").status().await.unwrap();
+        assert!(status.success());
+    });
+}